Conventional liquid developer for electro photography are agents in which an organic or inorganic pigment or dye such as carbon black, nigrosine or phthalocyanine blue, etc. and a natural or synthetic resin such as an alkyd resin, an acrylic resin, rosin or a synthetic rubber, etc., are dispersed in an aliphatic petroleum hydrocarbon or similar highly electrically insulating, low dielectric constant liquid and with additionally polarity control agent such as a metal soap, lecithin, linseed oil, a higher fatty acid or vinyl pyrrolidone, etc.
The resin in these types of developer is dispersed in the form of insoluble latex grains with a diameter of several nm - several hundred nm. In a conventional liquid developer, since the bonding between the latex particles which are insoluble in the liquid developer and a dispersion stabilization resin or the polarity control agent which are soluble in the agent is imperfect, the soluble dispersion stabilization resin and the polarity control agent are in a form in which they are easily dispersed in the solution. As a result, there is the drawback that on long-term storage or repeated use the soluble dispersion stabilization resin becomes detached from the insoluble latex grains, the grains precipitate, aggregate and accumulate and the polarity becomes unclear. Further, since it is difficult to redisperse the grains once they have aggregated and accumulated, the grains adhere all over to the development unit, and this leads to damage to the image portions and to development unit problems such as solution feed pump blockage, etc.
A means for achieving chemical bonding of the soluble dispersion stabilization resin and insoluble latex grains in order to minimize this drawback is disclosed in U.S. Pat. No. 3,990,980. However, although the dispersion stability as to natural precipitation of grains is improved to some extent in such a liquid developer, the improvement is still unsatisfactory. Moreover, when the developer is put into and used in an actual development apparatus there are the drawbacks that toner adhering to various portions of the apparatus hardens as a film, redispersion is difficult and apparatus malfunction and fouling of images, etc. occur. Further, there is insufficient redispersion stability for practical purposes. In addition, in manufacturing the above-noted resin grains, if monodisperse grains with a narrow grain size distribution are to be produced, there are great restrictions in terms of the combinations of dispersion stabilization agents and insolubilized monomers that can be used and generally grains with a broad grain size distribution containing a large amount of coarse grains or polydisperse grains in which two or more average grain diameters are present are produced. Also, since it is difficult to achieve the desired average grain size in monodisperse grains with a narrow grain size distribution, large grains of 1 .mu.m or more or very fine grains of 0.1 .mu.m or less are formed. Problems occur in that the dispersion stabilization agents used have to be manufactured using complicated and time-consuming manufacturing steps.
To eliminate these drawbacks, methods in which the degree of dispersion, re-dispersibility and storage stability of grains are improved by using insoluble dispersion resin grains in the form of copolymers of monomers that are insolubilized and monomers containing long-chain alkyl portions or monomers containing two or more polar components are disclosed in, e.g., JP-A-60-179751 and JP-A-62-151868 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Also, methods for improving the degree of dispersion, re-dispersibility and storage stability of grains by using insoluble dispersion resin grains in the form of copolymers of monomers containing long-chain alkyl portions and monomers that are insolubilized in the presence of polymers for which bifunctional monomers have been used or polymers for which a macromolecular reaction has been used are disclosed in, e.g., JP-A-62-166362 and JP-A-63-66567.
Furthermore, recently, methods of printing a large number of sheets, for example, 5,000 or more, using a master plate for offset printing by an electro photographic system have been attempted and particular advances have been made in improvements of master plates with the result that it has become possible to print 10,000 or more large-size sheets. Progress has also been made in connection with shortening of operation time in electrophotographic plate making systems and improvements have been made in speeding-up of the development - fixing stages.
With dispersed resin grains manufactured by the means disclosed in the above-noted JP-A-60-179751, JP-A-62-151868, JP-A-62-166362 and JP-A-63-66567, there is still failure in always achieving satisfactory performance in terms of dispersibility and re-dispersibility of the grains when the speed of development is increased or in terms of printing resistance in the case of large-size (e.g., size A3 or larger) master plates or with shortened fixing times.